The invention relates generally to magnetic disk data storage devices, and more particularly to controlling a mechanical arm used in the disk storage device.
As it is known in the art, computer systems generally include a central processor unit and a random access memory which are coupled together via a system bus. In order to store large quantities of data for use by the computer system it is known to use a magnetic disk storage device. Magnetic disk storage devices typically include a plurality of disks which rotate at a preselected speed, and an elongated flexible mechanical member commonly called an actuator. The actuator moves in a radial direction across the rotating disks in response to a drive signal which feeds a motor that controls its movement. Magnetic read/write heads at the end of the actuator are used to read and write data on the disk.
As computer processor speeds increase, there is a concomitant need to decrease access times to information stored on mass storage devices such as hard disk drives. In order to access information quickly, it is desirable for the actuator of a disk drive to be able to change its position rapidly.
In order to achieve swift motion of the actuator, it has generally been the accepted approach to feed time optimal square wave drive signals to the motor. In the art, such square wave signals are known as "bang-bang" forcing functions. One problem with these "bang-bang" forcing functions is that, rather than improve access time, they can have a detrimental effect on access times due to the characteristics of the components of the disk drive.
In particular, the read/write heads in the disk drive are attached to thin flexible cantilever beam structures or arms to provide in combination an actuator. An actuator arm has several mechanical flexible modes. The broad-banded power spectrum associated with the "bang-bang" square wave input command drive signal is likely to excite some of these modes, resulting in radial perturbations in head position relative to the track to be written or read. These perturbations may be large enough to delay the access or storing of data until the movement of the head decreases to an acceptable range. In addition, energy resulting from exciting these modes may fall within the human acoustic range and may be unpleasant to the ear.
One technique used to avoid the excitation of flexible modes was mentioned in a publication entitled "Minimizing Residual Vibration for Point-to-Point Motion", by P. H. Meckl and W. P. Seering. This technique provided a method of constructing an input forcing function by adding separate ramped sinusoids of selected frequencies to approximate a square wave input. The frequencies of the ramped sinusoidal functions are selected to control the frequency spectrum of the input forcing function. One problem with this approach is that the shape of the resultant forcing functions is very complicated. The necessary amount of data storage or computation time required to produce these complicated forcing functions renders them impractical for disk drive applications, particularly where fast access times are necessary. Another problem with this approach is that the shape of the resultant class of inputs is necessarily symmetrical. This may result in less than optimal performance, since the rotation of the voice coil type motor caused by the input drive signal causes a back EMF voltage which affects portions of the input drive signal differently.
Another approach to pre-shaping command inputs is described in a publication entitled "Preshaping Command Inputs to Reduce Telerobotic System Oscillations" by N. C. Singer and W. P. Seering. According to this method, an input forcing function is provided which is a linear combination of delayed versions of the original (in this case the "bang-bang") input forcing function. The resonant modes are excited by the series of delayed command inputs. By properly choosing the delays, however, the phase of the resulting vibrations will cancel. One problem with this method is that although allowing the cancellation of specific resonant modes at particular chosen frequencies, it is not practical for cancelling many modes, especially those whose resonant frequencies vary with time. In addition, this class of input signals is not necessarily band limited; thus, higher frequency modes which cause a significant portion of the acoustical noise may still be excited.